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Ca hardness and plasticizers

OK, sorry if this is a no-brainer, but I just have to make sure.
A fellow swimteam mom who works for a pool store told me that hardness is important for vinyl pools for maintenance of the plasticizers in the vinyl. Since I had to take both parts of organic chemistry twice , I don't have the confidence to argue the point with her, but I can't see how Ca would help stabilize a polymer. Anyone think they can make an argument either way? My thinking (which may be flawed - see chemistry skills, above) is that destabilizing the vinyl with low hardness is poo. (Very scientific, I know). Anyone care to enlighten me why this is or is not possible?

Calcium has no effect on plasticizers but low pH does. The only reason that some calcium is needed is to provide some water hardness to keep foaming down (soft water will foam more redily than hard water.) There is some evidence that calcium MIGHT be needed in fiberglass pools to prevent cobalt leaching out of the gelcoat and forming cobalt spotting.

This is actually standard pool store rhetoric to sell unnecessary chemicals. (I should know, I work in one!)